1. Field of Invention
The present invention relates to an illumination optical system and a projector.
2. Description of Related Art
In related art projectors, illumination light emitted from an illumination optical system is modulated by a modulation device, such as a liquid crystal panel, according to image information, and the modulated illumination light is projected onto a screen, thereby performing image display.
In order to achieve a uniform in-plane illumination distribution of illumination light to illuminate the modulation device, such a related art projector adopts an integrator illumination optical system in which light emitted from a light source is divided into a plurality of sub-beams, and the sub-beams are superimposed near the modulation device.
FIG. 7 is a schematic of a related art integrator illumination optical system.
The illumination optical system 1000 shown in FIG. 7 includes a light source device 1010 including a light source 1020, an elliptic reflector 1030, and an aspherical lens 1040. FIG. 7 also shows a first lens array 1050, a second lens array 1060, a superimposing lens 1070, and an illumination area LA, such as a liquid crystal panel. The optical components are arranged relative to a light-source optical axis 1020ax (center axis of light flux emitted from the light source device 1010). That is, the first lens array 1050, the second lens array 1060, and the superimposing lens 1070 are arranged substantially perpendicular to the light-source optical axis 1020ax so that the centers thereof are almost aligned with the light-source optical axis 1020ax. 
In the integrator illumination optical system 1000, the light source 1020 has a light-emitting portion (arc) having a certain length along the light-source optical axis 1020ax, and is placed so that the center of the arc is disposed near a focal point (first focal point) F1 closer to the elliptic reflector 1030 of two focal points of the elliptic reflector 1030 on the light-source optical axis 1020ax (the center axis of light flux emitted from the light source device 1010). Light emitted from the light-emitting portion is reflected by a reflecting surface 1030R of the elliptic reflector 1030, and the reflected light is changed into light substantially parallel to the light-source optical axis 1020ax by the aspherical lens 1040 while traveling toward a second focal point F2, and enters the first lens array 1050.
As shown in FIG. 7, the first lens array 1050 includes a plurality of cell lenses 1051 having a rectangular outline substantially similar to the shape of the illumination area LA and arranged in a matrix, and divides substantially parallel light from the light source device 1010 into a plurality of sub-beams by the plurality of cell lenses 1051. The second lens array 1060 also includes a plurality of cell lenses 1061 having a rectangular outline and arranged in a matrix, in a manner similar to that in the first lens array 1050, the cell lenses 1061 are provided corresponding to the cell lenses 1051 of the first lens array 1050, and the plurality of sub-beams emitted from the cell lenses 1051 of the first lens array 1050 are collected on the corresponding cell lenses 1061. The plurality of sub-beams emitted from the cell lenses 1061 of the second lens array 1060 are superimposed by the superimposing lens 1070 so as to illuminate the illumination area LA, such as a liquid crystal panel.
In this type of illumination optical system, when the parallelism of the light emitted from the light source 1020 is insufficient, the light cannot pass through the corresponding cell lenses 1051 and 1061 of the first lens array 1050 and the second lens array 1060. Accordingly, the related art includes a technique of increasing the parallelism of light emitted from the light source device 1010, which is disclosed in Japanese Unexamined Patent Application Publication No. 2000-347293, and which was invented by the present inventor.
On the other hand, some integrator illumination optical systems of such a type adopt a parabolic reflector that can simultaneously reflect and collimate light from the light source, without collimating light from the light source by the above-described combination of the elliptic reflector and the aspherical lens. However, in an illumination optical system using a parabolic reflector, as shown in FIG. 9, a reflecting surface 1080R formed of a paraboloid of revolution of a parabolic reflector 1080 has an entrance angle xcex8 (angle around a light-source optical axis 1020ax), which allows light beams radially emitted from a light source 1020 to be guided to an aspherical lens 1040, smaller than the entrance angle of the reflecting surface 1030R formed of an ellipsoid of revolution of the elliptic reflector 1030 (FIG. 9 shows a state in which light guiding to the aspherical lens 1040 is impossible because the entrance angle xcex8 is smaller than that of the reflecting surface 1030R). Therefore, the light utilization efficiency in the parabolic reflector is lower than that in the elliptic reflector 1030. Accordingly, the related art includes integrator illumination optical systems adopting elliptic reflectors.
In an illumination optical system using an elliptic reflector, however, the light intensity distribution is not uniform and tends to be biased toward the light-source optical axis, which causes the following problems. FIG. 10 shows a state in which the light intensity distribution is biased.
FIG. 10 illustrates the loci of light beams radially emitted from the center of a light source of a light source device using a related art elliptic reflector, and shows the illuminance distribution in which the illuminance is high in a center portion near a light-source optical axis 1020ax and decreases away from the optical axis. For this reason, in the related art illumination optical system 1000 shown in FIG. 7 adopting the elliptic reflector, although arc images 1062 formed on the second lens array 1060 should be contained in the cell lenses 1061, as shown in FIG. 11(a), they are offset in the center portion near the light-source optical axis 1020a, and extend into the cells on the peripheries of the cell lenses 1061 in which they should be contained, as shown in FIG. 11(b).
Light beams that are not contained in the cell lenses 1061 of the second lens array 1060, and extend therefrom cannot illuminate the illumination area and are useless, and this results in light loss. The light beams thus extending correspond to light beams that cannot pass through the corresponding lenses 1051 and 1061 of the first lens array 1050 and the second lens array 1060. Although the light beams emitted from the aspherical lens 1040 should be caused to pass through the corresponding cell lenses 1051 and 1061 of the first and second lens arrays 1050 and 1060 by increasing the parallelism of the light beams in the above-described related art illumination optical system, in actuality, some of the light beams in the center portion near the light-source optical axis 1020ax still cannot pass therethrough, and a solution to this problem is desirable.
The present invention addresses or solves the above and/or other problems, and provides an illumination optical system and a projector that can efficiently utilize light from a light source by directing at least light beams in a center portion around a light-source optical axis, of light beams emitted from an aspherical lens, slightly outward rather than parallel to the light-source optical axis so that the light beams can pass through the corresponding lenses of first and second lens arrays.
An illumination optical system according to one exemplary embodiment of the present invention includes a light source; an elliptic reflector to reflect light from the light source; an aspherical lens having a concave aspherical surface to substantially collimate light reflected from the elliptic reflector; and first and second lens arrays each including a plurality of cell lenses to divide substantially collimated light from the aspherical lens into a plurality of sub-beams. The light source, the elliptic reflector, the aspherical lens, and the first and second lens arrays are arranged along a light-source optical axis, such that, of the reflected light reflected by the elliptic reflector, the optical paths of at least light beams in a center portion around the light-source optical axis are changed to optical paths directed slightly outward rather than parallel to the light-source optical axis so that the light beams pass through the corresponding cell lenses of the first and second lens arrays.
In an illumination optical system according to another exemplary embodiment of the present invention, the conic constant that specifies the shape of the aspherical surface of the aspherical lens is set to be larger than a conic constant that specifies the shape of the aspherical surface of the aspherical lens that allows ideal parallel light to be emitted from the aspherical lens.
In an illumination optical system according to a further exemplary embodiment of the present invention, the paraxial curvature radius of the aspherical surface of the aspherical lens is set to be smaller than the paraxial curvature radius of the aspherical lens that allows ideal parallel light to be emitted from the aspherical lens.
In an illumination optical system according to a further exemplary embodiment of the present invention, the aspherical lens is shifted toward the elliptic reflector from the position at which it can emit ideal parallel light.
An illumination optical system according to a still further exemplary embodiment of the present invention uses the elliptic reflector in combination with the aspherical lens, in which elliptic reflector the distance between a first focal point and a second focal point is longer than that in the elliptic reflector that is combined with the aspherical lens so as to satisfy the condition to emit ideal parallel light from the aspherical lens.
According to these exemplary embodiments, light beams can pass through the corresponding cell lenses of the first lens array and the second lens array, and therefore, arc images of the light source formed on the second lens array can be contained within the cell lenses in which they should be contained. This makes it possible to reduce light loss and to efficiently illuminate the illumination area. (While it has been described that the arc images are formed on the second lens array, they may be formed on the second lens array or near the second lens array. This also applies to the following description.)
In an illumination optical system according to a still further exemplary embodiment of the present invention, of the reflected light reflected by the elliptic reflector, the light beams in the center portion around the light-source optical axis have the optical paths passing through four cell lenses around the light-source optical axis of each of the first lens array and the second lens array.
According to this exemplary embodiment, of a plurality of arc images formed on the second lens array, arc images in four cell lenses around the light-source optical axis, in which light loss is large because the arc images extend outside the cell lenses in the related art optical system, can be contained within the cell lenses, and therefore, it is possible to reduce light loss and to efficiently illuminate the illumination area.
A projector according to an exemplary embodiment of the present invention has any of the above-described optical systems for a projector.
According to this exemplary embodiment, a projector in which the illumination area can be efficiently illuminated and the brightness of a projected image can be increased can be achieved by incorporating any of the above-described illumination optical systems therein.